I believe its 7.5mm fuel hose. There is a fitted hose at the tank (not just bulk hose) which is about a foot long, then I'd presume they have another foot or so of hose for return at the tank. Everything else is under the hood.

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Jeff M.
Mercedes W123 DIY pages are now located here.
1983 / 1984 300D Sold
2000 CLK430 Cabriolet ~58k Sold
2005 Avalanche 4x4 ~66k